Journal of Engineering Geology

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With respect to development of underground structures and their high construction costs in intra- and inter-municipal transportation, it is necessary to study the stability of such structures. In this research, tunnel stability of Mashhad Metro line 2 with 17 km length is studied. First, the type of sediments and geotechnical properties in Mashhad Plain are investigated. The SPT profiles were prepared using Rock Work 2006 software. The soil classification tests and XRD results show that the soils in this line are mostly clay such as Illite and Kaolinite types. Afterwards, because most of the soils in this line are characterized as fine grained, the ground settlement using PLAXIS V8 software was performed. According to the numerical modeling and the depth of tunnel, the optimum depth for tunnel was determined.

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 The average of shear wave velocity of the upper 30 m has so far been one of the reliable parameters in seismic site classification in different building codes, despite the numerous weaknesses in the exact explanation of site dynamic characteristics. In this study, an empirical relationship is obtained between the average of the shear wave velocity of the upper 30 m and the average of the shear wave velocity of shallower depths, based on 79 shear wave velocity profiles, in Mashhad. This is followed by the recommendation of proper depths for the dynamic analysis of the site effect based on the information of shear wave velocity profiles and resonance period distribution in the investigated area. The depth of the S-wave velocity profile investigation, required for the analysis of deposit effects has been estimated more than 30 m. whith exception of the southern and western parts of Mashhad (adjacent hillsides). Such depth is estimated as about 80 m for central, eastern, and north-eastern areas, where the resonance period is more than 0.7 s. Therefore, investigation depth of 30 m is only adequate for site classification based on the building codes, and for theoretical analysis deeper studies is needed, in Mashhad

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In urban areas, it is essential to protect the existing adjacent structures and underground facilities from the damage due to tunneling. In order to minimize the risk, a tunnel engineer needs to be able to make reliable prediction of ground deformations induced by tunneling. Numerous investigations have been conducted in recent years to predict the settlement associated with tunneling; the selection of appropriate method depends on the complexity of the problems. This research intends to develop a method based on Artificial Neural Network (ANN) for the prediction of tunnelling-induced surface settlement. Surface settlements above a tunnel due to tunnel construction are predicted with the help of input variables that have direct physical significance. The data used in running the network models have been obtained from line 2 of Mashhad subway tunnel project. In order to predict the tunnelling-induced surface settlement, a Multi-Layer Perceptron (MLP) analysis is used. A three-layer, feed-forward, back-propagation neural network, with a topology of 7-24-1 was found to be optimum. For optimum ANN architecture, the correlation factor and the minimum of Mean Squared Error are 0.963 and 2.41E-04, respectively. The results showed that an appropriately trained neural network could reliably predict tunnelling-induced surface settlement.

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Introduction
Mashhad city, the second largest metropolis of Iran, is located in an arid and semi-arid region. Overexploitation of groundwater in Mashhad plain has caused up to 22.5-meter drop in the groundwater level from 1984 to 2013. The groundwater depletion in the unconsolidated aquifer has resulted in subsidence and cracks on the land surface. To determine the land subsidence rate map and the reasons for hot spot subsidence, the latest Envisat images of the ESA Space Agency's Archive for Mashhad plain were used. leveling and GPS data were combined with the radar interferometry results and the annual subsidence rate maps with high precision were obtained. Finally, the geology and soil texture maps of study area are compared to the land subsidence map.
Methods and results
To assess the land subsidence in Mashhad plain three methods of leveling, GPS and Insar are used. Leveling data are available in three profile of of Mashhad-Quchan (BCBD), Mashhad-Kalat (BDBE) and Mashhad-Sarakhs (BEBN) in two time interval of 1994-2003. The highest rates of subsidence in the BCBD, BDBE and BEBN lines are 7, 3.5 and 8.1 cm/year, respectively. Six permanent GPS stations have been installed in Mashhad plain, among them, NFRD, GOLM and TOUS have recorded the land subsidence, with the highest annual rate of 21.2 cm/year at TOUS Station. The third method applied to assess the history of land subsidence was InSAR radar interferometry which provided the extent and pattern of subsidence in all of the study area. For this, 23 images of the Envisat ASAR are processed during the 05/24/2010 to 06/30/2003 time period. The highest subsidence rate estimated by this method was 32 cm/year in the northwest of Mashhad. In general, two subsidence bowls, in the northwest and south east of Mashhad city are identified. Fig. 1 shows the annual subsidence rate map in Mashhad plain. Using the root-mean-square error (RMSE), the accuracy of the InSAR method was verified with GPS and leveling data.
Discussion
The rate and distribution of land subsidence in Mashhad plain are affected by geological factors such as soil texture, deposit thickness, geological structures and groundwater drawdown. The geological and geophysical studies and exploratory drilling results in the Mashhad Plain indicate that the bedrock morphology is very rough. The bedrock outcrops in some places while in some other places covered by more than 300 meters alluvial deposits. Generally, by distance from the mountain, alluvium thickness and as a result the likelihood of subsidence would be increased. Mashhad plain is surrounded by the active and quaternary faults in the north and south edges. In the north of Mashhad plain Marly bedrock is uplifted by Tous fault and outcropped in the north of fault. In the south of Mashhad two normal faults have resulted to the increase of alluvium thickness in south and central of Mashhad plain. The change of river pathway also let to deposition of a sequence of the fine-grained and coarse-grained soils in central of plain between Toos and southern branch of South Mashhad fault (F2).
used to draw the cross section
In order to evaluate the subsurface conditions and its effect on the land subsidence, the soil texture are studied using the deep water wells and piezometers log (Figure 2). Fig. 3 shows the longitudinal section (northwest to southeast) of the area. As it can be observed, the soil texture includes of alternation of fine and coarse grains layers (Figs. 4). In this condition, sandy soils help to shortening the drain path of clayey layers and leads to acceleration of the consolidation. The average rate of annual subsidence in the area is 14 cm for one meter of drop in the groundwater level.
Nowadays, in the urban area, due to the urban sewage waters, there is a rising of groundwater level.  Therefore, no land subsidence has occurred in the central parts of the city. It is expected by completion of urban sewage network about 62 million cubic meters of sewage water will be eliminated from the aquifer recharge, which will cause a notable drop in the groundwater level and prominent land subsidence in specific area of the city. Considering the geological conditions and the operation of the existing faults, it is expected that in the case of groundwater drop, no significant subsidence will occur in south of the F2 fault, due to the decrease in the alluvium thickness and to the coarse texture of the soil. But in the northern and northeastern parts of the city, which are located between F2 and the Tous faults, high rate of land subsidence is expected.
Figure 4: The cross section of soil texture and the annual average rate of land subsidence and groundwater level drop
Conclusions
Using the radar interferometry processing, the highest annual rate of subsidence in Mashhad plain is about 32 cm/year. Land subsidence in Mashhad plain has an increasing trend and the geological conditions have a critical role in the subsidence rate and its pattern. Generally, soil texture near the mountain area in South is coarse and grain size decreases toward the center of the plain. But because the outcrop of Marly formation in the north slopes, soil texture is mainly fine grains. In the center of Mashhad plain soil texture constituted of fine and coarse grains which are converted together as inter fingering facieses, which have a critical role in decreasing of the consolidation time and increasing the land subsidence rate. It is predicted by complimenting of the urban wastewater network, the groundwater level will be dropped in the city area and the northwest and southeast subsidence ellipsoids which already can be seen will be connected together. Therefore, the area between F2 and Toos faults, will be shown the highest rate of subsidence, due to high thickness and fine-grained soil texture../files/site1/files/133/5Extended_Abstracts.pdf

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Safety design of structures concerning surface faulting effects such as shear and differential subsidence are very costly and in some cases are impossible. Then the appropriate approach for encountering surface faulting is to determine a suitable fault-avoidance zone. In this study, firstly the theorem of avoidance fault zone is presented, and then the setback area from the fault zone of South Mashhad fault is proposed. Recent studies show that South Mashhad fault is a right-lateral strike-slip fault with a normal component that cut the Quaternary sediments. In this work, the average slip rate and estimated return period for South Mashhad fault are 0.59 mm/yr and 2930 years, respectively.  Accordingly, the proposed avoidance zones in the south (hanging-wall) and north (foot-wall) of the fault are 80 and 70 meters, respectively. Considering the avoidance zones, many residential and other important structures are located in the avoidance zone of the South Mashhad fault../files/site1/files/152/%D8%AD%D8%A7%D9%81%D8%B8%DB%8C.pdf